A typical wireless communication network includes a number of base stations each radiating to provide one or more coverage areas or “sectors” in which to serve wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices, whether or not user operated. In turn, each base station may sit as a node on a core access network that includes entities such as a network controller, switches and/or gateways, and the core network may provide connectivity with one or more external transport networks such as the public switched telephone network (PSTN) and the Internet. With this arrangement, a WCD within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the base station.
Such a system may operate in accordance with a particular air interface protocol, examples of which include, without limitation, Long Term Evolution (LTE) (using orthogonal frequency division multiple access (OFDMA) and single-carrier frequency division multiple access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WIFI), and BLUETOOTH. Each protocol may define its own procedures for registration of WCDs, initiation of communications, handover between sectors, and other functions.
Further, each sector may provide wireless service to WCDs on one or more carrier frequencies, with the air interface defining various downlink and uplink channels. For instance, on the downlink (in the direction from the base station to WCDs), the air interface may define a reference channel for carrying a reference signal that WCDs can measure to evaluate base station coverage strength, the air interface may define various other downlink control channels for carrying control signaling to WCDs, and the air interface may define one or more downlink traffic channels for carrying bearer data and other information to WCDs. And on the uplink (in the direction from WCDs to the base station), the air interface may define an access channel for carrying WCD access requests to the base station, the air interface may define one or more other uplink control channels for carrying control signaling to the base station, and the air interface may define one or more uplink traffic channels for carrying bearer data and other information to the base station.
When a WCD initially enters into coverage of such a system, the WCD may scan for a strongest sector in which to operate, and the WCD may then engage in signaling with base station that provides that sector, to register for service with the network. The WCD may then be served by the network a connected mode in which the WCD has an established radio-link-layer connection in that sector and the WCD and base station may exchange bearer data (e.g., application-layer communications), or in an idle mode in which the WCD does not have an established radio-link-layer connection in that sector but the WCD monitors the sector for page messages and may engage in signaling with the base station to transition to the connected mode.
When so served, the WCD may also regularly monitor coverage strength of its serving sector and of other sectors that extend to the WCD's location, to help ensure that the WCD is served with sufficiently strong coverage and perhaps with the strongest available coverage. If the WCD's coverage of its serving sector becomes threshold weak and if coverage of another sector becomes threshold strong (e.g., threshold stronger than the serving sector's coverage or than a defined level), then the WCD may then hand over to the other sector (e.g., through signaling with its serving base station, or autonomously).